Burke's justification for intervention in French internal affairs in the name of the international community has formed a powerful strand of thought in both diplomacy and international relations theory. However, the strength and openness of Burke's advocacy, traced here, changed according to his target audience, the domestic, and the international political context. Crucially, when he came to justify the case openly, the arguments changed completely. Beginning with a Grotian argument drawn from Vattel and premised on states as isolated rights-holders in a pre-social ‘state of nature’, Burke always struggled to draw a justification for intervention in the case, allowed by Vattel, of irrevocable political disunion. This conflicted both with Burke's general conception of states as corporate wholes and his linked policy aspiration to restore the totality of French ancient institutions. Ultimately abandoning this, his final argument, fully set out only in the Letters on a regicide peace, is completely new. It is premised not on modern international law but on remedies to be found in Roman domestic law, invocation of which he justifies by claiming Europe to be a single juridical enclave, drawing on an eighteenth-century discourse of shared manners, law, and culture as constitutive of political identity and community.